The present invention relates to a switching power supply apparatus for supplying a regulated dc voltage to an industrial or consumer electronic appliance.
In recent years, the demand has been increasing greatly for switching power supply apparatus which is smaller in size, more stable in output and higher in efficiency, as electronic appliances decrease in size, price, high performance and power-conserving design advances.
As an example of a prior art switching power supply apparatus that addresses such requirements, a full bridge converter will be elucidated with reference to FIG. 6. The drawing of FIG. 6 is a circuit diagram showing the configuration of the prior art full bridge converter.
In FIG. 6, an input dc power supply 111 is connected between input terminals 112a and 112b. A first switching device 121a and a second switching device 122a are connected in series between the input terminals 112a and 112b, and are turned on alternately, with a duty ratio below 50% interleaving therebetween, by control signals supplied from a control circuit 171. A third switching device 123a and a fourth switching device 124a are connected in series between the input terminals 112a and 112b. The third switching device 123a is controlled so as to turn on and off repetitively with the same timing as the second switching device 122a. The fourth switching device 124a is controlled so as to turn on and off repetitively with the same timing as the first switching device 121a.
A parasitic capacitor is formed in parallel with each of the first switching device 121a, second switching device 122a, third switching device 123a, and fourth switching device 124a. In FIG. 6, the respective parasitic capacitors are shown as capacitors 121c, 122c, 123c, and 124c.
A transformer 131 has a primary winding 131a, a first secondary winding 131b, and a second secondary winding 131c. The turns ratio of the primary winding 131a, the first secondary winding 131b, and the second secondary winding 131c is n:1:1. A first terminal of the primary winding 131a of the transformer 131 is connected to a connection point between the first switching device 121a and the second switching device 122a. A second terminal of the primary winding 131a of the transformer 131 is connected to a connection point between the third switching device 123a and the fourth switching device 124a.
The operation of the prior art full bridge converter will be described below with reference to FIG. 7. The drawing of FIG. 7 is a waveform diagram for explaining the operation of the full bridge converter according to the prior art.
In FIG. 7, G1, G2, G3, and G4 are the control signals supplied to the first to fourth switching devices 121a, 122a, 123a, and 124a, respectively.
In FIG. 7, V122 indicates the voltage applied to the second switching device 122a, V124 indicates the voltage applied to the fourth switching device 124a, and V131a the voltage applied to the primary winding 131a of the transformer 131.
In FIG. 7, I131a indicates the current flowing in the primary winding 131a of the transformer 131, I121 indicates the current flowing in the parallel circuit consisting of the first switching device 121a and the capacitor 121c. And, the waveform of I122 represents the current flowing in the parallel circuit consisting of the second switching device 122a and the capacitor 122c. To indicate the variation over time of the operating condition, time is plotted on a time scale of T0 to T4 in FIG. 7.
At time T0, when the first switching device 121a and the fourth switching device 124a are simultaneously turned on by the control signals G1 and G4 from the control circuit 171, the voltage V131a being applied to the primary winding 131a of the transformer 131 becomes the input voltage Vin. Voltage V131b on the first secondary winding 131b of the transformer 131 and voltage V131c on the second secondary winding 131c both becomes a voltage Vin/n.
As a result, a diode 161 is turned on and a diode 162 is turned off, so that voltage V163 across a third inductor 163 is a voltage Vin/n-Vout. Further, the sum of the magnetizing current in the primary winding 131a of the transformer 131 and a primary side converted current of the current flowing in the third inductor 163 flows into the first switching device 121a. The primary side converted current is the component such that a current flowing in the third inductor 163 is converted into the current flowing through the primary winding 131a. However, at time T0, at the instant when the first switching device 121a changes from the OFF state (nonconductive state) with a voltage Vin/2 applied thereto to the ON state (conductive state), the discharging of the capacitor 121c and the charging of the capacitor 122c occur instantaneously. This causes a spike current to flow, as shown with I121 of FIG. 7.
At time T1, when the first switching device 121a and the fourth switching device 124a are simultaneously turned off, the secondary current in the transformer 131 flows being split between the first secondary winding 131b and the second secondary winding 131c so that no discontinuity is caused in the magnetizing energy of the third inductor 163. At this time, the diodes 161 and 162 are both ON, and the voltages V131b and V131c on the first and second secondary windings 131b and 131c both become zero.
The voltage V163 across the third inductor 163 is then a voltage -Vout. Further, at the instant that the first switching device 121a and the fourth switching device 124a are turned off, an unwanted resonant voltage such as shown in V131a in FIG. 7 occurs due to leakage inductance of the transformer or energy stored in inductance parasitizing on wiring.
At time T2, when the second switching device 122a and the third switching device 123a are simultaneously turned on, the voltage V131a being applied to the primary winding 131a of the transformer 131 becomes the voltage -Vin. Then, the voltages V131b and V131c on the first and second secondary windings 131b and 131c of the transformer 131 both become a voltage -Vin/n. As a result, the diode 161 is turned off and the diode 162 is turned on, and the voltage V163 across the third inductor 163 becomes a voltage Vin/n-Vout.
At this time, the sum of the magnetizing current in the primary winding 131a of the transformer 131 and the primary side converted current of the current flowing in the third inductor 163 flows through the second and third switching devices 122a and 123a. The primary converted current is the component such that a current flowing in the third inductor 163 is converted into the current flowing through the primary winding 131a. Further, at time T2, at the instant when the second switching device 122a and the third switching device 123a are simultaneously turned on, spike noise occurs, just as at time T0.
At time T3, when the second switching device 122a and the third switching device 123a are simultaneously turned off, the secondary current in the transformer 131 flows being split between the first secondary winding 131b and the second secondary winding 131c so that no discontinuity is caused in the magnetizing energy of the third inductor 163.
As a result, the diodes 161 and 162 are both turned on, and the voltages V131b and V131c on the first and second secondary windings 131b and 131c both become zero. At this time, the voltage V163 across the third inductor 163 is a voltage -Vout. Further, at time T3, at the instant when the second switching device 122a and the third switching device 123a are simultaneously turned off, an unwanted resonant voltage occurs, just as at time T1.
At time T4, when the first switching device 121a and the fourth switching device 124a are simultaneously turned on, the voltage V131a being applied to the primary winding 131a of the transformer 131 becomes the input voltage Vin. This action is the same as that at time T0, and the operation from time T0 to time T4 is thus performed repeatedly.
The duty ratio of the first to fourth switching devices 121a, 122a, 123a, and 124a are set so that each ON period is equal such that T1-T0=T3-T2=Ton and each OFF period is equal such that T2-T1=T4-T3=Toff. By so setting, if it is assumed that, in steady state operation the magnetic flux of the third inductor 163 is in its initial state when the first switching device 121a is turned on. The following relation (1) holds since the magnetic flux returns to its initial state in one cycle period from the turn-on of the first switching device 121a to the next turn-on thereof. EQU (Vin/n-Vout).times.Ton-Vout.times.Toff=0 (1)
Hence, the output voltage Vout is related to the input voltage Vin by
Vout=.delta..times.Vin/n (2),
where .delta. in equation (2) is expressed by EQU .delta.=Ton/(Ton+Toff) (3).
That is, the output voltage Vout can be regulated by adjusting the duty ratio of each of the first to fourth switching devices 121a, 122a, 123a, and 124a.
Since current flows in the first to fourth switching devices 121a, 122a, 123a, and 124a in a balanced manner and thus, stress is distributed, the full bridge converter of the prior art has the feature of being able to be readily applied to large-power handling power supplies despite its compact size.
However, in the above-mentioned prior art full bridge converter, when the first to fourth switching devices 121a, 122a, 123a, and 124a respectively turn on, the charging and discharging of the associated parasitic capacitors occur instantaneously, causing a surge current. The prior art full bridge converter, therefore, has had the problem that power loss and noise are caused by this surge current.
Furthermore, when the first to fourth switching devices 121a, 122a, 123a, and 124a respectively turn off, an unwanted resonant voltage is induced due to leakage inductance of the transformer or parasitic inductance on wiring. This unwanted resonant voltage also has caused power loss and noise in the prior art full bridge converter.